1. Field of the Invention
The present invention relates to a computer system to which an extended unit can be connected and which has a modem board used as communication unit.
2. Description of the Related Art
In the field of personal computers, so-called laptop computers, which are small, light and portable, are used in increasing number. A laptop computer comprises a system body and standard peripheral units located within the system body, such as a keyboard, a liquid-crystal display, and a floppy disk drive. It is most desirable that a laptop computer be as small as possible. It is therefore preferable that as less input-output devices as possible be incorporated in the system body. This is why a communication unit, which enables the computer to exchange data with other computers, is not located within the system body in most cases. Rather, the communication unit is located outside the system body and electrically connected to the system body.
Lap-top computers have been developed, to which extended units (i.e., peripheral units other than the standard ones) can be connected. Each of the extended units has an extension board having a printer port, various serial I/O ports, and the like. The extension board of each extended unit is connected to the system body of the computer.
Further, laptop computers have been developed, to which extended units can be connected and which have a modem board built in the system body. Once any extended unit is connected to the computer, it becomes possible to utilize the various functions of the board built in the extended unit. Of course, the computer can receive data from, and transmit data to, any other computer, by means of the modem board built in its system body. The modem board is controlled, independently of the extended unit connected to the computer. In other words, the modem board is controlled, no matter whether or not the extended unit is connected to the computer. Hence, when a base address (I/O address) is set for the serial port of the modem board, with the extended unit connected to the computer, it may be identical to the base address set for the serial port of the extended unit. If this happens, neither the built-in modem board nor the extended unit can be used at all.